Contention-based networks, including local area networks such as Ethernet.RTM. networks, were originally designed with bursty data transmission in mind. More recently, such networks have been looked toward to support real-time applications such as voice and video. However, using contention-based networks to carry real-time traffic may be problematic because of such networks' inability to guarantee a maximum, or bounded, delay service and thus their inability to guarantee a desired level of service quality for real-time applications.
In order to address this issue, techniques have been proposed which give a certain degree of priority to real-time traffic. One approach is a pre-emptive approach wherein a station desiring to transmit a real-time packet simply comes on to the network's communications medium, e.g., coaxial cable, and transmits a fixed-length preamble. This causes a collision with any then existing traffic. Pursuant to conventional protocol schemes, the station that was transmitting the existing traffic ceases transmitting upon detecting that the collision occurred. At that point the real-time station which caused to collision simply continues with the transmission of its real-time packet. While serving to provide immediate access for the real-time station, this approach has the disadvantage that any packet that was being transmitted at the time of the collision will have to be retransmitted, thereby reducing the overall efficiency of the network. Moreover, the probability of such a collision increases with increasing network load, thereby increasing the number of collisions and further reducing efficiency at higher network loads.
A more efficient approach is that described for example, in I. Chlamtac, "An Ethernet Compatible Protocol for Real-Time Voice/Data Integration," Computer Networks and ISDN Systems, 10 (1985), Elsevier Science Publishers B.V. (North-Holland), pp. 81-96, in which the various currently active real-time stations establish an order of transmission among themselves and use, for example, the notion of a succession, or chain, wherein the real-time stations transmit one after the other in a way which does not allow the medium to be perceived by the non-real-time stations as being idle. It is only after all the real-time stations have had an opportunity to transmit packets they then have ready that the medium again becomes idle, and thus available to any station then having data to send. See also, for example, Szabo, "An Ethernet compatible protocol to support real time traffic and multimedia applications," Computer Networks and ISDN Systems, Vol. 29, page 335-42, 1997.
The nature of real-time traffic is that packets carrying that traffic originating from a particular node, once initiated, will continue over an extended period of time at regular intervals. In order to ensure robust operation, each of the real-time stations, having transmitted a packet, sets a timer which is to expire after another such interval has elapsed. If the timer of particular node expires without that node having transmitted a packet, remedial action needs to be taken. For example, a non-anticipated interruption may occur in that a station that is somewhere in the middle of the linked-list chain fails to transmit at the appropriate time, i.e., upon the termination of the transmission of the previous station in the list. This may be due, for example, to some kind of failure, or at least an untoward delay, in the application that is generating real-time data for the station, or there may have been a problem with the station itself, e.g., a hardware failure. The station which follows the failed station in the linked list chain will not transmit until it "sees" a transmission from the latter and thus if nothing more were done, neither that station nor any of those following ones--referred to herein as the "detached stations" are able to transmit, since they are waiting to observe that their predecessor in the chain has transmitted. However, expiry of a timer alerts its station that something untoward has occurred and recovery procedures are taken which have as their ultimate result the reconstruction of the linked list chain with those stations still active and wanting to transmit. If the network were to use a pre-emptive approach for the real-time traffic (meaning that a real-time station having a packet to transmit is allowed to access, and ultimately gain control of, the medium even though a non-real-time packet is in progress) the timer of the first detached station after the break would be the first and only one to have expired before recovery actions are initiated. As a result, each of the other detached stations would observe a transmission on the part of its predecessor in the linked list chain before its own timer expires and would therefore access the medium in the normal way after that predecessor has finished transmitting.